Hollow airfoil cavity surface texture enhancement

ABSTRACT

A method and passage system is applicable to hollow structures, such as hollow rib core fan blades, for removing alpha case and contamination, for improving surface texture, and for smoothing diffusion bonded joints to reduce notches and flaws, after fabrication. The passage system allows for venting and full cavity access to permit cleaning of the interior of the hollow structure. The method of cleaning the interior of a hollow structure may comprise applying an etchant to strip the topical layer from the inside of a hollow mechanical structure, such as a hollow core airfoil; or may comprise pumping an abrasive flow through the hollow cavities of the structure.

This application is a division of application Ser. No. 08/043,110, filedApr. 5, 1993, now U.S. Pat. No. 5,391,256.

BACKGROUND OF THE INVENTION

The present invention relates to aircraft engine systems and, moreparticularly, to a use of a chemical solution or abrasive flow to stripthe topical layer from the inside of a hollow mechanical structure suchas a hollow core airfoil of a high bypass turbofan engine.

Modern high bypass turbofan engines incorporate wide chord fan bladesfor improved aerodynamic efficiency. To minimize system weight, fanblade airfoils and dovetails are made hollow either by material removal,or by fabricating hollow constructions by forming and joining one ormore pieces into a final assembly. Titanium alloys are typically used infront-end turbomachinery static and rotating airfoils.

Since many of the manufacturing processes involve exposing the structureto elevated temperatures for long periods of time, the Titaniumoxidizes, or forms an alpha case structure on exposed surfaces. The hightemperatures from the thermal processes cause formation of the alphacase layer which is hard and brittle, and can thereby lead to failures.While this condition can readily be removed on the exterior of thecomponent, interior cavities are not accessible to traditional materialremoval or surface enhancement methods. The alpha case, or embrittledmaterial layer, is known to have negative effects on materialproperties, particularly on fatigue strength, an attribute critical torotating blades.

The joining and forming techniques used to produce hollow parts alsocontribute to the creation of many localized flaws and defects insidethe hollow parts which cannot be removed once the part is fabricated.These flaws act as stress raisers which can effectively reduce thefatigue strength of the component, and can result from mating surfacemisalignment or offsets, edge damage prior to bonding, poor stop-offdefinition, contamination, or tearing due to excessive forming rates.Such conditions are undesirable and detrimental to the integrity of theblade or vane. Furthermore, fatigue crack initiation and propagation isa major cause of failure in turbomachinery flow path hardware. Achievingthe highest possible fatigue strength of each part, particularly ofcritical rotating hardware, is crucial to the overall reliability andintegrity of the machine.

It is therefore highly desirable and an object of the present inventionto produce a final part void of internal alpha case and surface defectswhich degrade the fatigue properties of the structure.

Another object of the present invention is the use of an etchant, acid,or abrasive slurry on the interior of a fabricated part to overcome thedisadvantages of the prior art.

Another object of the present invention is to allow venting and fullcavity access in the interior of a fabricated part to maximize the useof the etchant, acid, or abrasive slurry on the interior of the part.

Another object of the present invention is to recontour local, sharpdiscontinuities that occur at bond lines.

These objects and other features and advantages will become more readilyapparent in the following description when taken in conjunction with theappended drawings.

SUMMARY OF THE INVENTION

The present invention has been developed to fulfill the needs notedabove. The present invention applies etchants, acids, or abrasiveslurries to hollow blades or vanes manufactured by super-plastic-formingand/or diffusion bonding to enhance the internal fatigue characteristicsof the structure by material removal and/or contouring.

Briefly, in accordance with the present invention, a method and passagesystem is applicable to hollow structures, such as hollow rib core fanblades, for removing alpha case and contamination, for improving surfacetexture, and for smoothing diffusion bonded joints to reduce notches andflaws, after fabrication. The passage system allows for venting and fullcavity access to permit cleaning of the interior of the hollowstructure. The method of cleaning the interior of a hollow structure maycomprise applying an etchant to strip the topical layer from the insideof a hollow mechanical structure, such as a hollow rib core airfoil.Alternatively, the method of cleaning the interior of a hollow structuremay comprise introducing an abrasive flow through the hollow cavities ofthe structure.

In the drawings as hereinafter described, a preferred embodiment isdepicted; however, various other modifications and alternativeconstructions can be made thereto without departing from the true spiritand scope of the invention. For example, the process can be applied tostatic and rotating airfoils, or to any hollow parts where small amountsof material removal are required on internal surfaces. The generalconcept can also be extended to other metals or materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative fan blade to which the process of the presentinvention may be applied;

FIG. 2 is a cross-sectional view taken along line 2--2 of the fan bladeof FIG. 1, illustrating alpha case and bonding defects present in theprior art;

FIG. 3 is a cross-sectional view taken along line 2--2 of the fan bladeof FIG. 1, illustrating elimination or correction of the defects of FIG.2, after application of the process of the present invention;

FIG. 4 is a bottom end portion of the representative fan bladeillustrated in FIG. 1; and

FIGS. 5-9 are sectional airfoil views taken along lines 5--5, 6--6,7--7, 8--8, and 9--9 of the bottom end portion of the fan blade of FIG.4, illustrating the transition from holes to cavities in the lower partof the blade of FIG. 1, introduced to allow access to each interiorcavity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, in FIG. 1 there is illustrated arepresentative fan blade 10 to which the concept of the presentinvention may be applied. The concept of the present invention involvesusing etchants, acids, liquid honing, or abrasive flows to removeresidual surface contamination, or improve flaw profiles on internalsurfaces of hollow pieces, particularly hollow aircraft blades andvanes, such as blade 10 of FIG. 1.

The fan blade 10 includes an airfoil 12 having a hollow core 14. Forpurposes of description only, the fan blade 10 of FIG. 1 is shown as arib core fan blade having ribs 16. Obviously, the concept of the presentinvention will apply to any hollow structure, including a truss core fanblade or other hollow core airfoil.

The fan blade 10 of FIG. 1 is typically constructed by joining aplurality of pieces to form the single structure 10 having a hollowinterior portion. The pieces may initially have slots machined thereinto form cavities 18 within the hollow interior portion 14 upon joiningof the pieces. The joining process may be any suitable joining process,such as diffusion bonding, wherein gas pressure or mechanical loading isapplied to the outer surface of the joined pieces. The plurality ofjoined pieces may comprise two, such as for a rib core; three, such asfor a truss core; or even four or more. The joined pieces are thensuperplastically formed to final shape. That is, the structure 10 is putin a hard die to re-inflate the slots or cavities, which may bedepressed by the diffusion bonding. Unfortunately, the thermal processesfor forming the structure 10 can cause formation of an alpha case layer.In addition, the joining and forming techniques used to produce hollowparts contribute to the creation of many localized flaws and defectsinside the hollow parts, such as at bonding surfaces.

Referring now to FIGS. 2 and 3, and continuing with FIG. 1, sections 20aand 20b of FIGS. 2 and 3, respectively, taken along line 2--2 of FIG. 1,illustrate internal cavities 22a and 22b, respectively, of the formedfan blade 10. The internal cavity 22a of section 20a of FIG. 2illustrates the alpha case and bonding defects inside the hollow parts,before the concept of the present invention is applied. As seen in FIG.2, an alpha case layer 24 or other contamination has formed on theinternal surface of cavity 22a. In addition, two pieces 26 and 28, whichcomprise the plurality of pieces joined to form the rib core fan blade10 of FIG. 1, illustrate notches 30 which are a sharp defect at bondingsurface 32.

The present invention eliminates these defects, as illustrated in FIG.3, where the alpha case layer 24 is gone and the notches 30 are smoothedout at bonding surface 32. In one embodiment of the present invention,an acid or etchant agent is admitted into the cavities 18 of thefabricated part 10, once all joining and forming is complete. Theetchant or acid displaces and removes the thin, relatively uniform alphacase layer 24 that may have formed on internal cavity surfaces. Theetchant or acid agent also smooths out or recontours any sharp flaws ator near bond lines, such as notches 30, which act as stress raisers. Thetype of chemical, solution mixture, and exposure time depends on thematerial, construction and amount of material removal necessary toachieve the desired effects. In one embodiment, an electrical potentialmay be induced between the cleaning agent fluid and the part 10 toinsure the rounding of sharp projections, such as notches 30, andaccelerate the removal process of alpha case layer 24.

In an alternative embodiment of the present invention, an abrasive flowor slurry is pumped through the cavities 18 to remove the alpha caselayer 24 and smooth out bond line flaws 30. This abrasive jet machiningprocess requires that a relatively high fluid velocity be achieved inorder to maximize the effectiveness of the process. In the abrasive flowor slurry process, material such as alpha case layer 24, is removed byfine abrasive particles, such as aluminum oxide or silicon carbide,which are carried in a high velocity stream of air, nitrogen, or carbondioxide. A slurry in which the abrasive is carried in a liquid medium,called liquid honing, may also be used.

In order to achieve the internal cavity cleaning process, the presentinvention includes a passage means for allowing access to the interiorof the hollow structure 10. The passage means comprises full cavityaccess at a bottom 34 of the structure 10, although the full cavityaccess may alternatively exist at the top of the structure. The fullcavity access is illustrated in FIGS. 1 and 5. The passage means furthercomprises cross-cavity venting at the opposite end, here top end 36, toallow through-flow in the structure 10 for high speed slurry cleaning.

Referring now to FIGS. 4-9, the full cavity access feature permits theetchant or the abrasive flow to be introduced to entrance ports 38, bestillustrated in FIG. 5, at the dovetail 34 of the structure 10. Theentrance ports 38 comprise the full cavity access of the presentinvention. Once introduced to the cavities 18, the etchant or slurry cantravel or flow upward in the direction of upward projecting arrows 40 inFIG. 1. The etchant or slurry then flows around each rib 16, asindicated by arrows 42 where the cross-cavity venting feature occurs,and expelled from the hollow interior 14 via discharge ports 44, in thedirection of downward projecting arrows 46. The cleaning agent, whichmay be the etchant or the slurry, travels through each cavity toeliminate the alpha case and bond line defects caused by the fabricationof the structure 10. As illustrated in FIGS. 6-9, the entrance ports 38and discharge ports 44 of FIG. 5, which exist at the very bottom end ofdovetail 34 of the structure 10, begin to assume a squared cavity shape22b of the rib core 20b, illustrated in FIG. 3, as the top end 36 of thestructure 10 is approached. It should be noted, however, that the shapeof the cavities may be any of a variety of shapes.

The present invention involves using a cleaning agent, such as anetchant or a slurry, to remove residual surface contamination andimprove flaw profiles in internal surfaces of hollow structures. Thepresent invention also addresses access for the cleaning agent into thehollow interior of the structure. For example, in a fan blade, access isprovided through the bottom or dovetail of the part. As will be obviousto those skilled in the art, these access channels could be filled, oncethe cavity surface enhancement process is complete, using plugs orinserts. For the abrasive flow technique, which requires an entrance anda discharge port, a serpentine or u-shaped internal flow path can beincorporated, as illustrated and described above.

It is seen from the foregoing, that the objectives of the presentinvention are effectively attained, and, since certain changes may bemade in the construction set forth, it is intended that matters ofdetail be taken as illustrative and not in a limiting sense.

What is claimed is:
 1. A method of cleaning the interior of a hollowstructure, the method comprising the steps of:joining a plurality ofpieces to form a single structure having a hollow interior portion;superplastically forming the plurality of pieces; and pumping anabrasive solution through the hollow interior portion at a high speed.2. A method as claimed in claim 1 further comprising the step ofproviding a passage means for allowing access to the interior of thehollow structure.
 3. A method as claimed in claim 2 wherein the step ofproviding a passage means comprises the steps of:providing full cavityaccess at one end of the hollow structure; and providing cross-cavityventing means at an opposite end of the hollow structure to allowthrough-flow of the abrasive slurry.
 4. A passage system for allowingaccess to the interior of a hollow structure, the system comprising:fullcavity access provided at one end of the hollow structure to allowaccess to the interior of the hollow structure; and cross-cavity ventingmeans at an opposite end of the hollow structure to allow through-flowof an abrasive slurry in the hollow structure.